This application is based on application Ser. No. 095929 filed in Japan on Mar. 29, 2001, the content of which incorporated hereinto by reference.
1. Field of the Invention
This invention relates to a battery charger for charging a plurality of batteries, and more particularly to a battery charger that controls charging current by detecting battery temperature.
2. Description of Related Art
Battery temperature rises during charging. Specifically, when batteries are charged continuously after being fully charged, the temperature of the batteries rises rapidly. Therefore, the fully charged status of the batteries can be detected by detecting the battery temperature, which makes it possible to stop charging when the batteries reach full charge. Further, the temperature of abnormal batteries may rise to an abnormally high level during charging. For this reason, it is possible to stop charging the abnormal batteries by detecting the battery temperature.
A battery charger, which detects fully charged batteries or abnormal batteries by temperature, is provided with temperature sensors for detecting battery temperature. The temperature sensors are disposed near each battery so as to detect the temperature of each battery. In a battery charger having a plurality of attachment channels for charging a plurality of batteries, temperature sensors are disposed near each battery so as to detect the temperature of each battery with precision. A battery charger having this structure requires the same number of temperature sensors as the number of batteries. For example, in a battery charger for charging four batteries, four temperature sensors are needed. For this reason, the battery charger has the drawback that the cost of production and parts becomes high due to the increased number of temperature sensors.
The drawback of the increased number of temperature sensors can be overcome by allowing a temperature sensor to detect the temperature of a plurality of batteries. In a battery charger that realizes this, one temperature sensor is disposed between adjacent batteries. In this battery charger, the battery temperature can be made lower than a charging stop temperature when all the attachment portions are provided with batteries. For example, a battery charger in which the charging stop temperature is set at 60 degrees centigrade can stop charging when the battery temperature reaches 60 degrees centigrade. However, in the battery charger having a plurality of attachment channels, all the attachment portions are not always set with the batteries when charging. In fact, in a battery charger having a plurality of channels for charging four batteries arranged in four rows, the battery charger did not stop charging until the battery temperature reached 65 degrees centigrade, even though the charging stop temperature was set at 60 degrees centigrade. In such a case, the two batteries were separately attached to attachment portions at both ends of the battery charger. If the batteries are charged until the battery temperature rises to an abnormally high level, their efficiency will be reduced. To avoid this harmful effect, the charging stop temperature is set at 55 degrees centigrade so as to stop charging if the temperature of the batteries at both ends of the battery charger has reached 60 degrees centigrade. Hence, when all attachment channels are provided with batteries, the battery charger stops charging if the battery temperature reaches 55 degrees centigrade. When the charging stop temperature is set at a low temperature, full charge cannot be attained, and the unwanted effect of stopping of charging normal batteries is caused.
The present invention was developed for the purpose of overcoming the above-described drawbacks. It is thus a primary object of the present invention to provide a battery charger having a plurality of attachment channels, that can definitely stop charging when the battery temperature reaches a set temperature, while using only a few temperature sensors.
The present invention is a battery charger for charging a plurality of rechargeable batteries attached in a detachable fashion, and is provided with temperature sensors for detecting the temperature of the batteries to be charged. The battery charger compares the battery temperature detected by the temperature sensors with a charging stop temperature, and stops charging if the detected temperature has exceeded the charging stop temperature. Further, the battery charger has a plurality of battery attachment detectors for respectively and independently detecting the presence of the batteries. The number of temperature sensors is made less than the total number of the batteries to be attached. In addition, the temperature sensors vary the charging stop temperature in accordance with the number of batteries detected in position by each of the attachment detectors.
The battery charger having this structure has the feature that it can definitely stop charging the batteries that have reached a set temperature, while using only a few temperature sensors. This is because the battery charger has a plurality of attachment detectors for respectively and independently detecting the presence of the batteries, and varies the charging stop temperature for stopping charging according to the battery attachment position detected by each of the battery attachment detectors. Since the battery charger changes the charging stop temperature to the most suitable temperature for stopping charging in accordance with the battery attachment situation, it can definitely stop charging when the actual battery temperature reaches a set temperature even though the number of temperature sensors is less than the total number of batteries. In this manner, the battery charger is capable of reducing the number of temperature sensors can be mass-produced at low cost due to cost reductions associated with parts and production.
The temperature sensor can be placed between two adjacent batteries. In this battery charger, the cost of production and parts can be reduced by cutting the number of temperature sensors in half. If the temperature detected by the temperature sensors has reached the charging stop temperature, the battery charger can stop charging by cutting off charging current. Further, it can significantly reduce charging by changing charging current to trickle charging current, if the detected temperature has reached the charging stop temperature. The battery charger is also provided with switching devices, connected in series to each battery, in order to control charging current, and can stop charging by moving the switching devices to an Off position, if the detected temperature has reached charging stop temperature.
The attachment detectors can detect battery presence by detecting voltage change of output terminals connected to the batteries. This configuration makes it possible to detect the presence of batteries with a simple circuit structure.
The above and further objects and features of the invention will be more fully apparent from the following detailed description in conjunction with the accompanying drawings.